Organic EL devices and operation thereof

Abstract

An organic electroluminescent device, particularly for use as a display, is disclosed which is driven according to a pulsed mode of operation which allows much higher current densities to be injected into the device. This is achieved by selection of particular pulse durations and duty cycles, and by an improved geometry for the electroluminescent device in which the resistance of anode lines is reduced.

What is claimed is: 
     
       1. An electroluminescent device comprising: a first set of planar electrodes extending in a first orientation;   a layer of an organic light emissive material arranged between the first and second electrodes;   first and second terminals provided respectively on the first and second electrodes; a second set of planar electrodes extending in a second orientation; and   circuitry adapted to apply a unipolar voltage pulse directly to the first and second terminals,   wherein application of the voltage pulse to the terminals causes the light emissive layer to generate light in an electroluminescent manner, and the resistance of the first set of electrodes is selected such that a peak current density of greater than 50 A/sqcm is manifest in the light emissive material at an applied voltage pulse of less than 90V.   
     
     
       2. An electroluminescent device according to claim 1, wherein the circuitry is configured to apply a sequence of voltage pulses to the first and second terminals with a duty cycle in excess of 1:500. 
     
     
       3. An electroluminescent device according to claim 1, wherein the first planar electrodes comprise anode strips, each comprising a layer of a charge injecting material, a conductive layer and an electrically insulating layer. 
     
     
       4. An electroluminescent device according to claim 3, wherein the charge injecting layer is selected from the group comprising indium tin oxide, fluorine doped tin oxide and aluminium doped zinc oxide. 
     
     
       5. An electroluminescent device according to claim 3, wherein the conductive layer is selected from the group comprising aluminium, copper, copper based alloys and aluminium based alloys. 
     
     
       6. An electroluminescent device according to claim 3, wherein the insulating layer is selected from the group comprising aluminum oxide, silicon oxide, silicon nitride, polyimide and spin-on glass. 
     
     
       7. An electroluminescent device according to claim 1, wherein the voltage pulse has a peak voltage lying in the range of 10-90V and a pulse duration of less than 1 μs. 
     
     
       8. An electroluminescent device according to claim 7, wherein the voltage pulse has a pulse duration of 0.25 μs. 
     
     
       9. A method of operating an electroluminescent device comprising first and second electrodes and arranged therebetween a layer of an organic light emissive material, the method including applying a sequence of unipolar voltage pulses with a duty cycle in excess of 1:500 directly across the first and second electrodes to cause the light emissive layer to generate light in an electroluminescent manner.   
     
     
       10. A method according to claim 9, wherein each voltage pulse has a peak voltage lying in the range of 10-90V and a pulse duration of less than 1 μs. 
     
     
       11. A method according to claim 10, wherein the pulse duration of each voltage pulse is 0.25 μs. 
     
     
       12. An electroluminescent device comprising: a first set of planar electrodes extending in a first orientation;   a layer of an organic light emissive material arranged between the first and second electrodes;   first and second terminals provided respectively on the first and second electrodes; a second set of planar electrodes extending in a second orientation; and   circuitry arranged to apply a unipolar voltage pulse directly to the first and second terminals,   wherein application of the voltage pulse to the terminals causes the light emissive layer to generate light in an electroluminescent manner, and the first electrode comprises a plurality of planar anode strips extending in a first orientation and the second electrode comprises a plurality of planar cathode strips extending in a second orientation, each anode strip comprising a charge injecting layer, a conductive layer and an insulating layer whereby the resistance of the anode strips is selected so that a peak current density of greater than 50 A/sqcm is manifest in the light emissive material at an applied voltage of less than 90V.   
     
     
       13. An electroluminescent device according to claim 12, wherein the charge injected layer is selected from the group comprising indium tin oxide, fluorine doped tin oxide and aluminium doped zinc oxide. 
     
     
       14. An electroluminescent device according to claim 12, wherein the conductive layer is selected from the group comprising aluminium, copper, copper based alloys and aluminium based alloys. 
     
     
       15. An electroluminescent device according to claim 12, wherein the insulating layer is selected from the group comprising aluminium oxide, silicon oxide, silicon nitride, polyimide and spin-on glass. 
     
     
       16. An electroluminescent device according to claim 12, wherein the voltage pulse has a peak voltage lying in the range of 10-90V and a pulse duration of less than 1 μs. 
     
     
       17. An electroluminescent device according to claim 16, wherein the pulse duration of each voltage pulse is 0.25 μs. 
     
     
       18. A method of operating an electroluminescent device comprising first and second electrodes and arranged therebetween a layer of an organic light emitting material, the method comprising: applying a sequence of voltage pulses, each having a pulse duration of less than 1 μs, with a duty cycle in excess of 1 to 500 directly across the first and second electrodes to cause the light emissive layer to generate light in an electroluminescent manner.   
     
     
       19. An electroluminescent device comprising: first and second electrodes;   a layer of an organic light emissive material arranged between the first and second electrodes;   first and second terminals connected respectively to the first and second electrodes; and   voltage application circuitry connected to the first and second terminals and configured to apply a sequence of unipolar voltage pulses with a duty cycle in excess of 1:500 directly across the first and second electrodes to cause the light emissive layer to generate light in an electroluminescent manner.   
     
     
       20. An electroluminescent device according to claim 19, wherein the voltage application circuitry is configured to apply each voltage pulse with a peak voltage lying in the range of 10 to 90V and a pulse duration of less than 1 μs. 
     
     
       21. An electroluminescent device according to claim 20, wherein the voltage application circuitry is configured to apply each voltage pulse with a pulse duration of less than 0.25 μs. 
     
     
       22. A method of operating an electroluminescent device comprising first and second electrodes and arranged therebetween a layer of an organic light emissive material, the method comprising: applying a unipolar voltage pulse directly across the first and second electrodes to cause the light emissive layer to generate light in an electroluminescent manner, the applied voltage pulse having a voltage of less than 90V, whereby a peak current density of greater than 50 A/sqcm is generated in the light emissive material.